The present invention relates generally to the field of amplifiers, and in particular, to transimpedance amplifiers for generating output characteristics having high bandwidth, high gain, low noise and stable operation by controlling the input impedance of the transimpedance amplifier.
Receivers in optical communication systems often use photodetectors to detect incoming optical signals. Photodetectors generate a current based on received optical energy. The current generated by the photodetector is then generally converted to a voltage using a transimpedance amplifier, namely an amplifier whose output voltage is dependent on its input current. In most commercial applications, the transimpedance amplifier operates at input currents between 1 .mu.A to 2 mA, and produce outputs on the order of 5 mV to 500 mV. Thus, the transimpedance amplifier generally has a small signal gain on the order of 5 k.OMEGA., i.e. 5V/mA. Minimal distortion of the signal spectrum is preferred, and therefore, bandwidth must be high, by way of example, 2 GHz for SONET OC-48 applications. Moreover, a high gain transimpedance amplifier is generally used, and the transimpedance amplifier is positioned at the front end of the receiver. Accordingly, noise due to the transimpedance amplifier should be kept to a minimum, on the order of 5 pA/Hz, in order to reduce noise propagation through the receiver. In addition, if a feedback architecture is used, stable operation of the transimpedance amplifier should be assured.
A conventional transimpedance amplifier is shown in FIG. 1. A photodetector 11 is used to convert an optical input into a current. The photodetector 11 is connected to the input of a transimpedance amplifier 13 with an open-loop gain of (A). A feedback resistor 15 is connected across the input and output of the transimpedance amplifier 13. The photodetector and the transimpedance amplifier have associated inherent capacitance. Accordingly, for analytical purposes, a shunt capacitor 17 representing the sum of the photodetector capacitance, the input capacitance of the transimpedance amplifier 13 and the parasitic capacitances is shown.
In operation, an optical signal entering the photodetector 11 is converted into a current I.sub.IN. Due to the high input impedance of the amplifier, the current I.sub.IN, for all intents and purposes, also flows through the feedback resistor. Thus, a voltage, V.sub.OUT, is developed at the output of the transimpedance amplifier 13 due to the flow of current I.sub.IN in the feedback resistor 15. Analysis of the closed loop system in the frequency domain yields Equation 1 for the transfer function from I.sub.IN to V.sub.OUT. ##EQU1##
From the transfer function of Equation 1, it can be seen that the dominant pole, (A+1)/(R.sub.F C.sub.IN), is set by the value of resistance R.sub.F of the feedback resistor 15, the input capacitance C.sub.IN, and the open-loop gain of the transimpedance amplifier 13 (A). For large open-loop gains, i.e., large values of (A), the dominant pole, and therefore, the corner frequency at which signal attenuation can be expected, is approximated by A/(R.sub.F C.sub.IN). Thus, from Equation 1, high closed-loop gain can be achieved by selecting a large value of resistance R.sub.F for the feedback resistor 15. However, for large values of R.sub.F, the amplifier gain (A) must also be high so that dominant pole can be maintained at a relatively high frequency so that high bandwidth may be achieved as well.
These constraints, however, can result in stability problems because the separation in frequency of the dominant pole from the pole of the amplier is reduced. Generally speaking, a closed-loop system is stable if the magnitude of the loop gain is less than unity when the phase shift is 180.degree.. Initially, the gain begins to roll-off at the dominant pole, and a 45.degree. phase shift is seen at the roll-off frequency. The phase shift continues to increase from the dominant pole as a function of frequency at a rate of 45.degree./decade. A further increase in gain roll-off is seen at the pole of the amplifier, and an additional 45.degree. of phase shift is added into the loop. The phase shift continues to increase from the pole of the amplifier as a function of frequency at a rate of 90.degree./decade. Set against this background, it can be seen that, for stability purposes, the poles should be sufficiently spread in frequency such that an increase in phase shift due to the dominant pole in conjunction with an increase in phase shift due the pole of the amplifier is such that at unity gain the phase shift of the loop is less than 180.degree..
Another important consideration is noise. Noise tends to be inversely proportional to the value of the feedback resistor 15 and is given by the expression in Equation 2, wherein T is the absolute temperature in degrees Kelvin, k is Boltzmann's constant, and .DELTA.f is the change in frequency. ##EQU2##
Accordingly, the noise can also be reduced by increasing the resistance value R.sub.F of the feedback resistor. However, as discussed above, for large values of resistance, the open-loop gain (A) of the transimpedance amplifier 13 must be increased proportionately to maintain sufficient bandwidth.
Conventional amplifiers, however, generally have a fixed gain-bandwidth product. Accordingly, increasing the open loop-gain (A) of the transimpedance amplifier 13 decreases the open-loop bandwidth of the transimpedance amplifier 13. In turn, this causes the pole of the transimpedance amplifier 13 to move toward the dominant pole of the closed-loop system. Accordingly, increasing both R.sub.F and the open loop gain reduces the phase margin, possibly resulting in instability. For low frequency applications where the gain-bandwidth product of the transimpedance amplifier 13 is well in excess of the desired system bandwidth, the approach of increasing both R.sub.F and the open loop gain works quite well. However, for high frequency applications, the phase margin problem can become catastrophic, and the solution of lowering the closed-loop gain increases the noise, or alternatively, if the closed-loop gain is not reduced, decreases the bandwidth. This may not always be possible for a given application. Therefore, there is a current need for a transimpedance amplifier with high bandwidth, high gain, low noise and stable operation.